In the past, avionics displays have been called upon to perform in extreme environmental conditions. The typical cockpit display device needs to be visible in a bright sunlit cockpit at 40,000 feet at noon and also viewable in a dark moonless cockpit at midnight. Consequently, it has been desirable to have a backlighted liquid crystal display with a wide dimming range to accommodate the extreme ambient light conditions.
Additionally, there has been a seemingly never ending quest for avionics engineers to extend the bulb life for fluorescent lamps (i.e. increase the time between bulb replacements).
In the past, dual filament lamps have been proposed such as in U.S. Pat. No. 4,734,616 entitled "Fluorescent Lamp with Double Cathode and Probe", which was issued to Donald P. Northrop on Mar. 29, 1988; which patent is incorporated herein in its entirety by this reference. This patent shows the use of dual filaments in order to extend bulb life. However, this and similar designs have several serious drawbacks.
First of all, these lamps are not dimmable over a wide range which is needed for avionics applications.
Secondly, when the first filament burns out in the Northrop design it causes some material to be deposited on the second filament.
Consequently, there exists a need for improvement in fluorescent lamps used in the avionics areas.